1. Field of the Invention
The present invention relates to a secondary air supply passage structure for use in an engine exhaust purification system.
2. Description of the Background Art
Japanese Patent Laid-Open Publication No. Hei. 9-324624 shows an engine with a reed valve chamber provided on a cylinder head. A secondary air supply passage connects the reed valve chamber and an exhaust port. The secondary air supply passage is roughly parallel to a cylinder axis of the cylinder head.
It is desirable that the secondary air supply passage be formed as close as possible to a right angle with regard to the head gasket surface. By forming the secondary air supply passage at a right angle, the secondary air supply passage can be formed by a machine processing. One drawback is that when the secondary air supply passage has an end passing through the exhaust gas port, it is highly likely that the secondary air supply passage will interfere with the cam area at the top of the cam shaft. Therefore, the process of forming the secondary air supply passage becomes extremely difficult. Furthermore, if a pinch angle between the valves is narrowed, the process becomes even more difficult.